The invention herein is generally concerned with the feeding of web material to high speed rotary treatment apparatus, particularly die cutting apparatus and the like. More specifically, the invention is concerned with an electromechanical computer controlled method and system for the elimination of web material waste between successive blanks treated or cut from the web while simultaneously maximizing the production rate from the die regardless of the die length.
In rotary die cutting, the impression or die cylinder and anvil roll are maintained at constant peripheral velocity during steady state operation. The web while engaged therebetween must move at the same forward velocity. However, since the die cylinder is of a fixed diameter and circumference, dies of differing length leave more or less of the die cylinder periphery unused for cutting so that if the web were continuously fed into the nip at a constant velocity, the web would not be cut into usable blanks as it passed through the unused portion of the die nip. Unless the feeding of the web is controlled to eliminate it, this would result in substantial waste of material passing between the die cylinder and anvil roll during passage through the gap between die passes even if the web were braked while in the gap due to the inherent tendency for the die to pull excess web. Some machines cutting web material on rotary die cutters simply put a drag brake on the web and run the press at low speed. Without pullback, they always experience a 3-4" strip of scrap between passes of the die caused by the inherent overfeed.
In U.S. Pat. No. 3,756,149, issued on Sept. 4, 1973, to Thomas Bishop, there is disclosed apparatus which, through cam controlled feed rolls, achieves a pullback of the web material during that gap or those periods of disengagement by the treatment means or die. While the web control achieved by Bishop 3,756,149 is significant and results in a substantial reduction in waste in a structurally simple and efficient manner, the Bishop apparatus does not provide for a complete elimination of waste between impressions of the trailing edge of the die and the subsequently encountered leading edge thereof on the next revolution. In addition, set-up for differing die lengths is a meticulous and time-consuming, therefore costly, operation. Also, the acceleration rates of the Bishop apparatus are constant, limiting operation to the necessary speed for the longest die lengths and thereby reducing production on short dies.
In U.S. Pat. No. 4,416,200, issued on Nov. 22, 1983, to William S. Yon, there is disclosed apparatus which improves on the Bishop apparatus and includes a cam controlled sequence of operation wherein the web engaged feed rolls, through an engaged clutch, are accelerated and forwardly driven at a constant velocity equal to the die-induced velocity of the web immediately before die engagement. The clutch is then disengaged and the web pulled through the nip by the engaged die. After the die disengages the web, cam drive is then used to cause the web to be pulled back while the die is clear of the web and then be re-accelerated in a forward direction so that upon reengagement of the die with the web, the web will again be traveling at the die velocity and in the die movement direction but with no wastage of web material. Generally, during engagement of the die and web, the feed rolls are declutched and the web is driven solely by the die, although unlike the Bishop apparatus, the feed rolls remain engaged with the web throughout the operating cycle. With the cam system of Yon, this declutching is necessary to enable the cams to reset their followers for the next pullback cycle and acceleration of the web. The acceleration and deceleration of the feed rolls and web, both before and after engagement with the die, and during the pullback part of the cycle, however, are fixed by the cam contour and are not under the operator's control. The amount of pull back is under the control of the set-up mechanic, within the limitation of the constant velocity portion provided in the cam, but achieving registration is still a time-consuming trial and error process and the mechanical cams, clutches, and followers require substantial lubrication and other maintenance. Obviously, the amount of pullback cannot be adjusted while the machine is running as adjustment requires moving the cams with respect to the die.
The longest die that can be accommodated is limited by the cam profiles to about 55" on a 66" circumference die drum, whereas many saleable products lie in the range 55"-60" in length. The new method accommodates dies up to 60" in length.
Further, since the distance over which acceleration and deceleration of the feed rolls occurs in fixed in both the Bishop apparatus and the Yon apparatus by the mechanical cam profiles and the length of the pullback is a fixed optimal amount, on all but the longest die a dwell period is necessary in the web feed cycle to compensate when shorter than maximum length dies are in service. This dwell and the fixed acceleration and deceleration rates result in less than maximum production rate being achieved when less than maximum length dies are in service. Since the acceleration and deceleration rates of the feed rolls of the Yon apparatus are preset by the cam contours, it is not practical to recalculate, remachine or change them with changes between dies of differing lengths. This fixed acceleration/deceleration rate also results in a single maximum die cylinder velocity for all die lengths thereby penalizing any shorter die production by restricting its speed to that allowable for the longest die.
A related or resultant problem to feeding, withdrawing and subsequently re-accelerating the web from a supply roll is not discussed or referred to in the Bishop patent and is merely touched upon by the disclosure of the Yon patent. This related problem is control of the supply roll and pull-off of web material therefrom in a smooth and constant manner while the web is being accelerated, decelerated, advanced, and retracted in a rapid and cyclical manner into and out of the treatment or die cutting means. The massive supply roll cannot be unwound and rewound in accordance with the needs of the feed rolls at the rates required for efficient operation. The Yon patent teaches provision of a slack or supply loop between pull or pull-off rolls which takes web material from the supply roll into the slack or supply loop and feed rolls which take web material from the slack or supply loop into the nip and back again to provide a low inertia buffer zone between the massive supply roll and the cutting die rolls or other treatment means. The driving of the pull or pull-off rolls in Yon is controlled by a set of photocells in a manner whereby the slack loop is maintained in the web beyond the pull rolls, but no specific circuitry or control means therefore is disclosed. In fact, in the Yon apparatus, the pull rate from the supply roll into the loop is manually set by the operator and the photocell detector system operates primarily as a backup to the operator's skill and judgment.
Establishing uniform pull-off of the web from the supply roll initially, however, requires substantial effort by the machine operator, especially during set-up and start-up because differing die lengths result in differing rates of material usage and hence a differing ratio of die cylinder speed to pull-off roll speed. All of these multiple factors must be monitored by the operator and/or set-up technician. The pull-off rolls are preferably kept to a uniform velocity related to overall per cycle usage of material at speed and for any die length so that the massive supply roll becomes part of a stable system without oscillation or other instabilities and need not accelerate and decelerate contantly throughout the cutting cycle each time the web velocity and direction of web travel through the nip changes.
The mechanical cam based systems of Bishop and Yon, therefore, while each providing substantial savings in raw material, still cannot provide optimal set-up, start-up or on-fly adjustment during running conditions. Set-up and start-up of these systems require a great deal of operator experience, effort and skill. Running conditions, while providing material economy better than previously available methods, apparatus and systems, still cannot practicably maximize production rates with differing die lengths since the feed roll cam profiles need to be designed to accommodate the longest die length to be used and cannot be readily changed for each change in die length and speed.